A good friend has a Kubota that I THINK is from the late '80's. He's been searching for a steering part for 3 years. Seems some of the earlier models changed so fast that parts were hard to keep up with.
From what I've seen (with local Kubota dealer) parts (on newer tractors) seem to be "easy enough" to get, just VERY pricey.
Over the long haul, I've found that most of the "urban myths" regarding parts prices are just that....myths. Deere parts aren't cheap, but by no means are they the highest. (My nod there goes to NH) I've also found Deere and AGCO/MF have the best record as far as parts availability. The only Kubota parts I ever needed were on the shelf at the dealer, so that wasn't an issue.
The entire issue seems to vary wildly from dealer to dealer, more so than brand to brand.
This is why I like my Massey Ferguson 150 so much! I never need parts for it!